1. Field of the Invention
The present invention relates to the downhole evaluation of formation fluids produced into a wellbore. More particularly, the present invention relates to a system that simultaneously combines conductivity and dielectric permittivity sensors in a single downhole module for real-time multiphase flow evaluation.
2. Background of the Art
Fluids are produced from a borehole drilled into the formation rock. The fluids are tested to evaluate the presence of hydrocarbons, the flow-rate, and the presence of multiphase fluids created by the combination of water and hydrocarbons. The resulting data provides information regarding the concentration of hydrocarbons in the formation. The data also provides information regarding the composition and location of hydrocarbons and suggests procedures for maximizing the completion and production of hydrocarbon reservoirs.
The composition of formation fluids can be identified by certain electrical characteristics. Hydrocarbon fluids have a low conductivity, while salt water fluids typically found in subsurface formations have a relatively high conductivity. Because of this fundamental difference in conductivity, downhole sensors measure the conductivity of the formation fluids. Relative conductivity is evaluated by measuring the amount of current transmitted through the formation fluid sample between two or more electrodes when a selected voltage is applied to source electrodes.
In addition to conductivity characteristics, hydrocarbon fluids have a different dielectric permittivity than salt water brines. Dielectric permittivity sensors are usually constructed as a capacitor and measure changes in the capacitor's dielectric. However, dielectric permittivity sensors cannot effectively operate in a conductive medium where the conductivity of the formation fluid exceeds 0.0001 S/m. This phenomenon occurs because the displacement currents between the capacitor plates become negligible when compared to galvanic currents in the formation fluid.
Although downhole sensors have been constructed to evaluate the dielectric properties of formation fluids, such sensors do not provide stable and accurate results when the fluids are electrically conductive. Measurement of dielectric permittivity and conductivity is complicated by the physical difficulty in measuring these parameters at low frequencies with the same electrodes. Accordingly, a need exists for an improved downhole sensor that can accurately and efficiently evaluate multiphase formation fluids.
U.S. Pat. No. 5,736,637 to Evans et al. discloses a system for evaluating multiphase flow of a fluid downhole in a borehole. Dielectric permittivity electrodes generate a capacitance output signal through the fluid, and conductivity electrodes generate a conductivity output signal through the fluid. The electrodes are powered with an AC generator operating at the same or different frequencies. The capacitance and conductivity output signals can be alternately generated by operating a controller, and such signals can be combined with a multiplexer engaged with the controller. The signal can be processed downhole or can be transmitted to a receiver positioned at the well surface for processing and interpretation of the multiphase data.
Maxit et al., “Downhole Instrumentation for the Measurement of Three-Phase Volume Fractions and Phase Velocities in Horizontal Wells”, discuss a downhole logging instrument that measures three-phase flow in horizontal and highly deviated wells. Signals from a two-dimensional array of capacitive sensors are measured, and subsequent processing determines the volume fraction and velocity of each component of the borehole flow. The design incorporates an array of capacitive sensors that span the wellbore. Since two orthogonal electrical signals are used to excite local fluid elements, it is possible to determine the fluid capacitance or the volume fraction of conductive fluid in the vicinity of each sensor. Holdups (concentrations of the different components of the multiphase fluid) are calculated from these sensor outputs. Velocities are calculated by correlating outputs from adjacent sensors. The device of Maxit does not require the complications of dual frequency measurements faced by the Evans device.
The Multi-Capacitance Flow Meter (MCFMSM) of Baker Hughes Incorporated is schematically illustrated in FIGS. 2a, 2b. The tool body is denoted by 101. The tool has a housing and is provided with wings 105. Holdup sensors are denoted by 103. Additionally, arrays of sensor for measuring velocity and holdup are denoted by 107. The tool measures holdups and velocities with alternating current from two transmitter electrodes driven in quadrature: a capacitive electrode and a conductive electrode. The capacitive transmitter electrode consists of an insulated metal plate, while the conductive electrode is in direct contact with the borehole fluid. On the inner side of the wing plate opposite the transmitter electrodes, there is a two-dimensional array of 26 capacitive sensors arranged as shown in FIG. 2a. Eight of these sensors, which span the diameter of the borehole, are used for holdup measurements.
In a horizontal well, the fluid flow profile is expected to be symmetric about a vertical plane that contains the borehole centerline. The MCFM measures velocities at seven levels in this plane, with six arrays of capacitive sensors, as shown in FIG. 2a, and a mechanical spinner in the center.
The basic concept of the MCFM measurement is shown in FIG. 3. The sensor electrode 319 and the film covering it 317 form the input capacitor Ci of a charge-coupled amplifier 303. The transmitter electrode 301 is driven with a sinusoidal voltage VT from a generator. The amplifier makes the sensor electrode a virtual ground for the transmitter, so current I flows between the transmitter and sensor electrodes. The galvanic current flow is denoted by 305, the conductance electrode by 311, 313 is a shield, 315 is a capacitance electrode, 307 denotes the displacement current. The capacitive transmitter electrode has a high output impedance while the galvanic transmitter electrode has a low output admittance. Feedback capacitor Cf allows the output voltage of the amplifier Vout to be proportional to the input current Iin. The complex voltages VT and Vout are recorded. The complex fluid admittance Y between the transmitter and sensor electrodes is determined as follows.
                    Y        =                                            I                              i                ⁢                                                                  ⁢                n                                                    V              T                                =                                                    -                                  j                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                                                                          ⁢                                              fC                        f                                            ⁢                      Vout                                        )                                                                              V                T                                      .                                              (        1        )            
The MCFM sensor measures the admittance of a volume of fluid in series with the capacitance of the film covering the electrode. The benefits of this film are threefold: a) enhances the resolution of liquid and gas phase hydrocarbons, b) makes the admittance of water less dependent on conductivity, and c) protects the metal electrodes from chemical interactions with borehole fluids. An important aspect of this is the phase detector 309.
The film covering the electrode also compresses the phase difference between conductive and non-conductive fluids. To distinguish between these fluids, the MCFM incorporates two transmitters driven in quadrature. A signal generator produces a sinusoidal voltage that drives two transmitter electrodes as shown in FIG. 3. The capacitive transmitter electrode 315 is insulated from the fluid, while the conductive transmitter electrode 311 is in galvanic contact with the fluid. The signal applied to the conductive transmitter electrode is shifted 90° relative to the signal on the other electrode. Additionally, the conductive transmitter electrode is shielded from the sensor electrode.
In oil or gas, current from the capacitive transmitter electrode is detected as the imaginary component of Vout. In this condition, the admittance between the conductive transmitter electrode and the sensor is so small that this current contributes little to Vout. In water, current from the conductive transmitter electrode is measured as the real component of Vout, while current from the capacitive transmitter electrode is shunted to ground by the low output impedance of the conductive transmitter. With this method, each holdup sensor produces a complex voltage: the real part is representative of the volume fraction of water, and the imaginary part is indicative of the capacitance.
The electrical admittance of a fluid represents its ability to conduct current in response to an electrical potential. There are two types of current: galvanic current and displacement current. If a fluid conducts galvanic currents, it has conductivity σ. If a fluid conducts displacement currents, it has a permittivity ∈. A volume of fluid has a conductance G proportional to its conductivity σ, and a capacitance C proportional to its permittivity ∈. These proportionality constants are functions of the fluid volume geometry as well as the geometry of the electrodes creating the electrical potential. Therefore, a measurement of current I, resulting from potential V at a frequency f, can be equated to a measurement of the conductivity G and the permittivity ∈ of a volume of fluid by the relation:
                              I          V                =                              G            +                          j              ⁡                              (                                  2                  ⁢                  π                  ⁢                                                                          ⁢                  fC                                )                                              =                                    σ              ⁢                                                          ⁢                              k                1                                      +                                          j                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    f                    ⁢                                                                                  ⁢                    ɛ                    ⁢                                                                                  ⁢                                          k                      2                                                        )                                            .                                                          (        2        )            
The electrical admittance is a complex number, since it must equate the magnitudes and phases of the current and voltage. The geometric factors, k1 and k2, relate the conductivity and capacity of a fluid, respectively, to the conductance and capacitance of a volume of fluid. The MCFM measures both the complex current and voltage between transmitter and sensor electrodes, thereby directly yielding the admittance of the fluid.
Hydrocarbons are non-conductive, while well water is generally conductive. Oil has a dielectric constant more than twice that of gas. Therefore, by measuring the electrical admittance of a volume of fluid, it is possible to identify the fluid. Whenever multiple fluids occupy a volume, the resulting admittance is a function of both the distribution and quantities of the individual fluids. The admittance of such a mixture of fluids will fall in the region bounded by the admittance of the individual fluids. Different models can be used to convert the measured admittance of the fluid into actual component holdups. This topic is well covered in the literature.
At MCFM's frequency of operation, the conductive component of the admittance of water is typically 4 to 6 orders of magnitude larger than the capacitive component, and 5 to 7 orders of magnitude larger than the capacitive component of the admittance of hydrocarbons. In order to compress the dynamic range of this response, each MCFM sensor electrode is covered by an insulating film, which lowers the admittance of these sensors.
A drawback of the prior art device is some complexity resulting from having to insulate the receiver plate and the transmitter plate.